We are into this as well.
The oldest grandson is 5, grandaughter is 4, and younger grandson is 3.
I told the oldest just as soon as he could ride his bicycle without training wheels, that we would buy them a little motorcycle.
So, I started working with him real hard for about a week and he did it.
Shortly thereafter I picked up the PW. Started him out on it all over again just like we did on the bicycle. In two days he was riding OK.
I have the throttle limit control screw turned way in and gradually backing it out as he gets better at handling the bike.
Starting out, I was having to run alongside him in case he spilled it so he wouldn't get hurt and then discouraged. He's doing much better now.
Almost immediately after watching the elder grandson, the granddaughter wanted her bicycle training wheels off too, so I am working with her now.
The three year old loves riding the PW with me holding it up for him and running alongside. He understands throttle control, but he can barely reach the foot pegs.
Like the guys said, if they can ride a bicycle without T-wheels, then they can learn a PW.
I began by standing out like 10 or 12 ft in front of him and having him focus on me as he rode towards me and I would catch him. Then I just kept standing further and further away.
We also have them in riding pants, jersey, chest protector, helmet and goggles so they grow comfortable with safety gear on.